Gamma-valerolactone (GVL), i.e. 5-methylbutyrolactone or 5-valerolactone, is a chemical compound that can for instance be used as a precursor in the manufacture of adipic acid, i.e. 1,6-hexanedioic acid. Adipic acid and 1,6-hexanediamine are the monomers in the preparation of polyamides, viz. nylon-6,6. Further, adipic acid can be used in the production of polyurethanes. Esters of adipic acid can be used as plasticizers, e.g. in PVC. Other applications of adipic acid include its use as gelling agent in foods and as ingredient in controlled release medicines.
Adipic acid may be produced from GVL by converting GVL to pentenoic acid or a pentenoic acid ester. By carbonylation in the presence of water or an alcohol pentenoic acid or an ester thereof may be converted to adipic acid or an adipate ester. The use of GVL as starting material is environmentally advantageous, as GVL can be made from renewable sources.
It has been known for a long time to prepare GVL from levulinic acid. U.S. Pat. No. 2,786,852 discloses a process for the preparation of GVL by passing a mixture of hydrogen and levulinic acid in the vapor phase over a copper oxide catalyst. The use of lower alkyl levulinate ester, in particular ethyl levulinate ester, is disclosed in U.S. Pat. No. 4,420,622, describing a process for the production of valerolactones from such esters and hydrogen in the presence of a solid particulate hydrogenation catalyst that contains a metal from Group VIII or Group Ib of the Periodic Table of Elements. The use of cobalt, nickel and copper on silica, magnesium oxide or chromium oxide is explicitly disclosed. Examples in U.S. Pat. No. 4,420,622 show that the use of levulinic acid instead of levulinate ester results in a fast deactivation of the catalyst. Both prior art processes are conducted in the gas phase. This entails that the reactor size is relatively large. Moreover, temperature control may be complicated and thermal gradient may be difficult to control. This all adds to the costs of such a gas phase process.
U.S. Pat. No. 6,617,464 discloses a process wherein levulinic acid and hydrogen are contacted with a catalyst that contains a Group VIII element, i.e. Groups 8-10, of the Periodic Table of Elements, in particular Ir, Pd, Pt, Re, Rh and Ru. The catalyst further contains a carrier, such as silica, titania, alumina and carbon. It may further contain a promoter, selected from another Group VIII metal or a Group Ib (Group 11) metal. The most preferred catalyst is ruthenium on carbon. The reaction is carried out in the liquid phase. The liquid phase is accomplished by dissolving levulinic acid in a suitable solvent, such as dioxane or 5-valerolactone.
The process according to U.S. Pat. No. 8,598,303 also employs levulinic acid as starting material and ruthenium on carbon as catalyst. The process is conducted in the presence of water, viz. up to 10% wt, based on the amount of levulinic acid. In U.S. Pat. No. 8,598,372 a gas phase reaction is described wherein levulinic acid is converted into GVL with hydrogen over a supported copper catalyst.
A supported copper catalyst was also used in the process described in U.S. Pat. No. 8,975,421. In this case levulinic acid was dissolved in water or methanol, or methyl levulinate was dissolved in methanol and reacted with hydrogen over a catalyst comprising copper on zirconia or copper on alumina. Especially the presence of levulinic acid and water caused significant metal leaching, and thus loss of catalytic activity.
In JP2014-166604 a process is described wherein levulinic acid or an ester thereof is contacted with hydrogen and a catalyst comprising copper oxide, zinc oxide and/or aluminum oxide. In this process the levulinic acid compound is in the gaseous phase, as it is believed that the hydrogenation of levulinic acid in the liquid phase requires high temperatures and noble metal-containing catalysts.
The influence of the catalyst support on the reaction between levulinic acid and hydrogen to GVL over a supported catalyst is described in US 2011/0046399. In this document a process is described wherein levulinic acid is contacted with a catalyst comprising one or more hydrogenation metals, such as those from Groups 8 to 10 of the Periodic Table of Elements, supported on titania or zirconia as carrier. These carriers provide more active catalysts than those which comprise silica or carbon as carrier. The levulinic acid is brought in the liquid phase by adjusting the reaction temperature and pressure. Although the description suggests that esters of levulinic acid can also be used, no proof thereof has been provided.
Another example of a process for the reaction of levulinic acid with hydrogen over a supported nickel catalyst is described in J. Lv et al., RSC Adv., 2015, 5, 72037. In this process levulinic acid is reacted with hydrogen in an autoclave in the presence of isopropanol as solvent. The support appears to have a significant influence on the conversion of the levulinic acid. Silica, alumina, titania, zirconia, zinc oxide and magnesium oxide were tested, wherein magnesium oxide as support shows the best effect. The highest conversion obtained in this batch process was 58.1% after two hours.
In U.S. Pat. No. 8,003,818 a process is disclosed wherein catalysts comprising a zeolite and a silica binder as support were compared with similar catalysts that did only contain silica as support. As hydrogenating metal component the catalyst may contain a variety of metals, in particular Groups 8 to 10 metals, such as nickel, rhodium, palladium, platinum, ruthenium, rhenium or combinations thereof. The catalysts were compared in the reaction of ethyl levulinate with hydrogen. It appeared that the catalysts that contained only silica as carrier produced certain amounts of GVL, whereas the catalysts that additionally contained a zeolite promoted the yield of pentenoate and pentanoate esters. This prior art document suggests that the presence of acidic groups may be detrimental to the yield of GVL.
Levulinate esters may also be converted into GVL by not using hydrogen as hydrogen donor, but by using an alcohol, such as isopropanol or 2-butanol as hydrogen donor. A metal oxide catalyst may be used. Such catalysts, such as zirconia, alumina, magnesia or titania convert the levulinate esters to GVL in the presence of the alcohol hydrogen donor. The basic sites on the solid catalysts may cause the formation of by-products such as condensation products. Such a reaction has e.g. been described in US 2012/0302764. This document also discloses the conversion of levulinic acid with hydrogen over a catalyst that comprises ruthenium as hydrogenating metal, tin as dopant and carbon as carrier. In US2007/0208183 another process is described wherein formic acid is used as hydrogen donor for the reaction of levulinate ester to GVL. The catalyst used can be a hydrogenating metal, such as nickel, on silica. The patent application describes that the reaction is preferably conducted in the gas phase. The process may be conducted batch-wise or continuously.
From the description of the prior art it is apparent that for the production of GVL from levulinic acid or from an ester thereof a variety of catalytically active metals can be used, and that the support that is used in the catalyst may have an influence on the catalytic activity. Further, the reaction may be conducted in the gas phase or the liquid phase, wherein the liquid phase may be provided by the use of a solvent or the application of suitable temperature and pressure. Moreover, an alcohol, formic acid or a hydrogen-containing gas may be used as hydrogen donor. When the process employs a levulinate ester as starting material, the prior art mainly discloses batch processes, which are less preferred than continuous processes.